networkmanager: unbreak build with latest kernel headers

| nm-ppp-manager.c: In function 'monitor_cb':
| nm-ppp-manager.c:307:23: error: storage size of 'req' isn't known
| make[5]: *** [libppp_manager_la-nm-ppp-manager.lo] Error 1

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>